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Go Constants


Go Constants

If a variable should have a fixed value that cannot be changed, you can use the const keyword.

The const keyword declares the variable as "constant", which means that it is unchangeable and read-only.

Syntax

const CONSTNAME type = value

Note: The value of a constant must be assigned when you declare it.


Declaring a Constant

Here is an example of declaring a constant in Go:

Example

package main
import ("fmt")

const PI = 3.14

func main() {
  fmt.Println(PI)
}
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Constant Rules

  • Constant names follow the same naming rules as variables
  • Constant names are usually written in uppercase letters (for easy identification and differentiation from variables)
  • Constants can be declared both inside and outside of a function

Constant Types

There are two types of constants:

  • Typed constants
  • Untyped constants

Typed Constants

Typed constants are declared with a defined type:

Example

package main
import ("fmt")

const A int = 1

func main() {
  fmt.Println(A)
}
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Untyped Constants

Untyped constants are declared without a type:

Example

package main
import ("fmt")

const A = 1

func main() {
  fmt.Println(A)
}
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Note: In this case, the type of the constant is inferred from the value (means the compiler decides the type of the constant, based on the value).


Constants: Unchangeable and Read-only

When a constant is declared, it is not possible to change the value later:

Example

package main
import ("fmt")

func main() {
  const A = 1
  A = 2
  fmt.Println(A)
}

Result:

./prog.go:8:7: cannot assign to A
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Multiple Constants Declaration

Multiple constants can be grouped together into a block for readability:

Example

package main
import ("fmt")

const (
  A int = 1
  B = 3.14
  C = "Hi!"
)

func main() {
  fmt.Println(A)
  fmt.Println(B)
  fmt.Println(C)
}
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